Solid waste can be divided into various fractions distinguished, among other things, by how easily they can be biodegraded. The organic fraction is the part of the waste that is most easily biodegraded and may also be referred to as organic waste. The organic fraction is primarily made up of food waste, but may also include leaf and yard waste or other materials. The organic fraction is approximately 40% of ordinary municipal solid waste (MSW) after recyclables are removed.
Historically, organic waste was landfilled with other solid waste. However, the organic fraction of solid waste is the major cause of greenhouse gas emissions, leachate and odors in landfills. There is a general trend to divert organic waste for biological treatment, for example by anaerobic digestion (AD) or composting. Most biological treatment steps require some preprocessing of the waste such as debagging and sorting to remove large items such as bottles and cans. Certain biological treatments, such as some composting methods and high-solids slurry and wet (low solids) anaerobic digestion, also require that the waste be reduced in size and homogenized. The size reduction is typically done in a device that comminutes the waste, such as a hammer mill, shredder or pulper. In some cases, the comminuting device also provides a coarse separation of contaminants (i.e. material that is not readily biodegraded, such as plastic). Alternatively, a separate separation device may be added.
Wet anaerobic digestion is typically performed in one or more mixed tanks. These systems are entirely contained and so allow for high levels of odor control and biogas recovery. In many cases, the organic waste can also be co-digested with wastewater treatment plant (WWTP) sludge by modifying existing WWTP digesters rather than building new facilities.
US Publication 2013/0316428 describes an alternative process in which an organic fraction containing biological cells is separated from solid waste in a press. The organic fraction is extruded through a grid having small-bore holes, under a pressure higher than the burst pressure of the cell membranes. The cells are disrupted and a gel or paste of a doughy consistency is produced. The gel can be digested in an anaerobic digester. The press may be as described in European Publication Nos. 1207040 and 1568478 and Italian patent publication ITTO20111068. In general, these presses use a plunger to compress waste that has been loaded into a cylinder. The sides of the cylinder are perforated with radial holes. US Publication 2013/0316428, European Publication Nos. 1207040 and 1568478 and Italian patent publication ITTO20111068 are incorporated herein by reference.